Currently asphalt used in paving roofing and the like is generally stored, moved and transported in a hot molten state since asphalt is solid and does not flow at ambient temperatures. Storing and shipping asphalt in the molten state is both costly and environmentally unfriendly. It would be desirable to provide a particulate asphalt product which is free-flowing at ambient temperatures since this would result in energy savings and would also avoid the accidents to workers which occur in the transporting of hot molten asphalt as well as the problems created by the unpleasant asphalt odours, and oxidative deterioration of the asphalt.
It has long been known that various non-viscous and viscous sticky materials can be successfully sprayed in a molten state to form spherical droplets. The droplets can then be cooled in an air tower or water-cooled (hydroprilling) tower to produce solidified droplets, or prills. The present inventor's U.S. Pat. No. 3,323,166 issued Jun. 6, 1967 is an example of such an apparatus used for prilling materials such as myristyl alcohol, coal tar resin caustic soda, wax plastic, hydrocarbon resin and coal tar pitch. Although it has often been shown that asphalt and like materials can be prilled, the difficulties encountered in handling prilled asphalt after it is fully solidified have blocked all attempts to produce a viable solid prilled asphalt product. Prills made from most asphalts are extremely sticky, even when dropped into a water bath for solidification, and agglomerate into an unmanageable mass.
The existing approach to solving the problem of agglomeration of asphalt prills has involved powdered coatings. See for example U.S. Pat. No. 3,016,568 J. Moar issued Mar. 27, 1962; U.S. Pat. No. 3,036,338 H. Nack issued May 29, 1962 and U.S. Pat. No. 4,107,382 H. Augustine et al. issued Aug. 15, 1978. Very fine powders of materials such as clays, line, talc, diatomaceous earths, and carbon black have been tried as coatings for the asphalt prills with varying degrees of success. Powder coatings alone, however, do not stand up over long periods of time, or under adverse climatic conditions, particularly where high ambient temperatures are present. The liquid asphalt from the prill slowly leaches through such coatings causing an increasing degree of agglomeration during storage and shipping. Further some coatings may have a negative effect on the asphalt product produced from the prills.
There is therefore a need for an asphalt prill which does not result in agglomeration of the asphalt prills during transporting or storage under the range of temperatures which might be encountered and in which the coating material enhances the properties of the asphalt end product.